3. If you only want to use Segoe UI Light when the web page doesn't specify a particular font (e.g. Wikipedia), use the same style as above, but delete both instances of !important

To Firefox, Segoe UI and Segoe UI Light are the same font family. The former is used for normal weight, while the latter is used for lighter weight.
* https://developer.mozilla.org/en-US/docs/CSS/font-weight
1. Install the Stylish add-on.
* https://addons.mozilla.org/firefox/addon/stylish/
2. If you want to ignore fonts specified by web pages and use Segoe UI Light for all elements if possible, paste the following in a new style and save it.
----
<pre><nowiki>@namespace url(http://www.w3.org/1999/xhtml);
@-moz-document url-prefix("http") {
* {
font-family: "Segoe UI" !important;
font-weight: 300 !important;
}
}</nowiki></pre>
----
3. If you only want to use Segoe UI Light when the web page doesn't specify a particular font (e.g. Wikipedia), use the same style as above, but delete both instances of '''!important'''

Question owner

Many thanks indeed for your helpful solution - J P D would be proud of you!

I particularly like the fact that (presumably) this approach could be used to apply other fonts and weights to Firefox.

@Gingerbread_Man
Many thanks indeed for your helpful solution - J P D would be proud of you!
I particularly like the fact that (presumably) this approach could be used to apply other fonts and weights to Firefox.